Yes and no.

When you do a fullscreen output to a custom position the underlying libraries end up squashing the image so that it all shows on the screen.

In your case, set the vmix canvas to 4k, add an 'Output' input, scale its height to be 4 times taller, send that out your fullscreen.

In practice this means you only use the top 1/4 of your canvas. I'd love for vmix to have custom resolutions but for now we're stuck at 16:9 for high resolutions.